The invention relates to a method and apparatus for processing video picture data for display on a display device. More specifically the invention is closely related to a kind of video processing for improving the picture quality of pictures which are displayed on matrix displays like plasma display panels (PDP) or other display devices where the pixel values control the generation of a corresponding number of small lighting pulses on the display.
The Plasma technology now makes it possible to achieve flat colour panel of large size (out of the CRT limitations) and with very limited depth without any viewing angle constraints.
Referring to the last generation of European TV, a lot of work has been made to improve its picture quality. Consequently, a new technology like the Plasma one has to provide a picture quality as good or better than standard TV technology. On one hand, the Plasma technology gives the possibility of xe2x80x9cunlimitedxe2x80x9d screen size, of attractive thickness . . . but on the other hand, it generates new kinds of artefacts which could degrade the picture quality.
Most of these artefacts are different as for CRT TV pictures and that makes them more visible since people are used to see the old TV artefacts unconsciously.
A Plasma Display Panel (PDP) utilizes a matrix array of discharge cells which could only be xe2x80x9cONxe2x80x9d or xe2x80x9cOFFxe2x80x9d. Also unlike a CRT or LCD in which grey levels are expressed by analogue control of the light emission, a PDP controls the grey level by modulating the number of light pulses per frame (sustain pulses). This time-modulation will be integrated by the eye over a period corresponding to the eye time response.
Since the video amplitude determines the number of light pulses, occurring at a given frequency, more amplitude means more light pulses and thus more xe2x80x9cONxe2x80x9d time. For this reason, this kind of modulation is also known as PWM, pulse width modulation.
This PWM is responsible for one of the PDP image quality problems: the poor grey scale portrayal quality, especially in the darker regions of the picture. This is due to the fact, that the displayed luminance is linear to the number of pulses, but the eye response and its sensitivity to noise is not linear. In darker areas the eye is more sensitive than in brighter areas. This means that even though modern PDPs can display e.g. 255 discrete video levels for each colour component R,G,B, the quantisation error will be quite noticeable in the darker areas. Further on, the required degamma operation in PDP displays, increases quantisation noise in video dark areas, resulting in a perceptible lack of resolution.
There are known some solutions which use a dithering method for reducing the perceptibility of quantisation noise in PDP""S.
From a former European Patent Application of the applicant having the number 00250099.9 it is known to use a 3-dimensional static dither patterns for the cells of a plasma display panel to improve the grey scale rendition. The three dimensions corresponding to a number of frames, a number of lines and a number of columns on the PDP. With this pattern for some video levels some checked patterns could be seen in homogenous surfaces that reduce the picture quality.
From EP-A-0 994 457 it is known to store a set of dither patterns in memory and to select the right dither pattern in dependence on time data (field data) and positional data indicating the locations of the pixels on the display panel. In another embodiment different dither patterns are used independence on what video level an input data word represents. The different video level ranges are e.g. defined to be 0 . . . 15, 16 . . . 31, 32 . . . 47, and 48 . . . 63.
From U.S. Pat. No. 6,069,609 it is known to store in memory a number of different dither patterns called xe2x80x98dither typesxe2x80x99. These dither patterns are selected with the 5 higher bits of an 8 bit input data word and the three lower bits determine which of the eight dither values from the dither type needs to be taken for dithering.
To overcome the drawback of reduced picture quality when using a static 3-dimensional dither pattern, the present invention, reports a dithering technique that makes use of different dither patterns for different entries in a number of least significant bits of the data word representing the input video level. In case that the dither patterns itself are also 3-dimensional, the invention utilizes quasi a 4-dimensional dither pattern with the fourth dimension being the entries in a number of least significant bits of an input video level data word. The invention makes it possible to suppress the disturbing patterns occurring on the plasma screen when using the 3-dimensional dither pattern.
Further advantageous embodiments are apparent from the dependent claims.
The input data words need to be transformed in a form where they have more bits than necessary for the final bit resolution that is required in the subsequent sub-field coding process. The bit resolution needs to be increased corresponding to the bit resolution for the dithering process.
This transformation can advantageously be done in a degamma calculation step in which the input video levels are amplified in order to compensate for the gamma correction in the video source.
In cases where the set of admissible input video levels is restricted, for the purpose of optimising the sub-field coding process by taking only those video levels to which sub-field code words are assigned that are less sensitive to the dynamic false contouring, the transformation can also be done in a resealing step where the data words for the reduced set of video levels are translated into data words having less bits.
For an apparatus for processing video pictures according to the invention it is advantageous when the dither unit comprises a table of different dither patterns for different entries in a number of least significant bits of the data word representing the input video level and a selector that assigns to a given video level one of the dither patterns stored in the table. This is a simple implementation of the invention.
Further advantageous is if for each colour component R, G, B of the input video signal a dedicated table of dither patterns is provided in the dither unit. This allows to use a single clock signal, e.g. pixel clock for dither number retrieval in the dithering process.